Amplifier



.April 21, -.1-936.

l.. E. BARTON AMPLIFIER original Filed Jan. so, 1932 S EG .Ennuh UQPatented Apr. 21, 1936 AMPLIFIER 'Loy E. Barton, Collingswood, N. J.,osslgnoito Radio Corporation of America, a corporation of DelawareOriginal application January 90, 1932, Serial Nor 589,846. Divided andthis application May 29, 1933, Serial No. 673,370. In Spain January 24.,

C l lsclaims.- (cl. ris- 1) static discharges have relatively little-eii'ect on My invention relates to amplifiers and, more particularly,to methods and apparatus for obtaining volume control in amplifiers ofthe type utilized in the reception of radio signals, and is a divisionof my copending application, Serial No. 589,846, filed January 30, 1932for-Amplifiers, and assigned to the same assignee.

Many automatic volume control systems, as

heretofore devised, have been open to some objection because of thenecessityfor providing` an additional thermionic tube to supply gridbiasing potentials varying in response to fluctuations in carrieramplitude. Furthermore, by reason of vthe fact that volume controlsystems, in

' general, permit maximum amplifier sensitivity when no carrier isbeingc'received, the background noise received when tuning from onestation to another has been excessivefand unpleasant.

In addition, automatic volume. control systems that respond tomodulation areopen to the objection that static, ifstrong,'causes`temporary paralysis of reception while the bias potentialresulting from the static is being dissipated. At-

tempts at rendering the systems unresponsive to static involve the useof circuits having timeconstants within the audio range. AIf circuitshaving time-constants too short are used, low

audio frequencies are effected, if too long, the recovery from a staticdischargel is slow.

The invention further contemplates the use of manual volumecontrol meansin the said amplifying system, and with the automatic volume to Iprovidean improved volume control systenr thatshall not be open to theforegoing enumerated objections. l

- It is a further object of my invention to provide an improved tonecompensated volume conr trol system for use in connection with audiofrequency circuits, which is simple and. effective,

and involves. comparatively inexpensive circuitv elements. t l

Preferably, in practising my invention, I consolidate the functions ofdemodulation of a signal and the supplying of a volume control potentialinto a single thermionic device, thus eliminating the customary extravolume-control tube.

The circuit is so arranged that the automatic volume control does notreduce the volume in a' volume or sensitivity. y

Since automatic volume control responds marily to' carrier, lowfrequency modulation or peak modulation does notaffect the automaticvolume control.

The manualvolume control means is located in an audio frequency circuit,"such as the detector output circuit, and is provided with tone signalattenuating circuits connected to suitably spaced taps thereon, togradually eilect a desired pri? tone control for predetermined volumelevels,

simultaneously with the adjustment of the volume. This. maybe designatedas an automatically tone compensated volume .control system.

The novel features that I consider characteristic of my invention areset forth with particularity' in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, 'together with additional objects and '.advantages'thereof,will ,best be understood from the following description of a-specifimembodiment' when read in connectionl with the accompanyingdrawing, which the single iigureis a diagrammatic view of a portion of'an amplierprovided with volume control means according to my invention.

In common with other automatic volume con- -trol systems I providebiasing means,.for one or more amplifying tubes, responsive to theamplitude of an incoming carrier-wave.A In my improved system,`,referring to' the drawing.' the biasing means includes a thermionictube I,

having a cathode 3, a plurality of grids 5, l, and 9, and an anode I I.l tube commercially designated RCA-238, is vsatisfactory.

one of alegrias, preferably the grid s, oom- Y monly used as a screengrid,is connected through -a resistor I3 to a source I5 of positivepotential.

.The grid 9, which serves as the input or control electrodef is`connected to th'ecathode through an inductor I1, -which may or may notbe provided with a tuning condenser l I9, and a fixed condenser 2|.

Normal bias for the thermionic device I, sufficiently negative to enableit to function satisfactorily as a demodulator, is provided byconnecting the control-grid 9 through a resistor 23,`

\to the junction 24 between the negative terminal of the high potentialanode-supply source and' a resistor 25 interposed between the saidterminal yand the ycathode of the device.

The potential dividers 25 and 26 acrossA the voltage supply furnish thenecessary potentialto The screen grid pentode keep the grid negativewith respect to the cathode.

The anode II of the thermionic device I is contype, having a cathode 33,a plurality of grids and 31, and an anode 39., i

The grid 35 .of the intermediate frequency amplifier tube is connectedtoLthe cathode thereof ythrough an inductor 4I, which may or may not vbeprovided with a tuning condenser 43, but includes a by-pass condenser45. The cathode 33 of the device is connected tothe junction point 24between the cathode of the/demodu'lator tube I and the aforementionedresistor 25.,7 through a self-bias-resistor 41.

The grid 35 of the intermediate frequency amplifier tube is connected tothe junction point 24 through a resistor 48 and the resistor 29 includedin the anode`circuit of the Ademodulator tube. Normal negative grid biaspotential for the intermediate frequency amplifier tube, therefore, issupplied by the drop across the self-bias resistor 41, the magnitude ofthe resistor being so chosen with respect to the plate and screen gridpotentials vsupplied 'that the said drop'is correct for maintaining theproper normal bias.

In the operation of my improved system, signals impressed upon thedemodulator tube from the previous amplifying stage are demodulatedtherein and appear as signals at audio frequency in the output circuitincluding the screen grid 5 and the resistor I3.' Since, as is wellknown to those` skilled in the art, the output current. from thedemodulator tube includes a component at the 70' upon frequency ofmodulation down to perhaps carrier frequency, the said component will bereimpressed between the anode and the cathode of the tube, by reason ofthe inductive couplingexisting between the output inductor 21 and an.inductor '49 connected in shunt relation to the -resistorl 3 insofarasthe carrier frequency is concerned. A small condenser 50, offering highim- `pedance to audiofrequency, is included in circuit with theinductori49. The lowimpedance ,of the vcircuitf49-5II to carrier, andhigh impedance to audio frequency result -in relatively low percentagemodulation ofthe carrier frequency induced in the inductor 21. 'I'hishas an important practical result, i. e., only smallamounts of staticfrequencies are re-impressed on the detector I. 'I'he carrier,thereafter, is again rectified in thedemoduylator tube, giving rise to auni-directional current which ows between the anode and the cathode ofthe device in the circuit including the output inductor 21, the resistor29, and the resistor 25.

Because of the high impedance of the resistor I3- t' audio frequencies,the energy or signal transferred to circuit 21 has only a limited degree`Vof modulation as compared to the original carrier. A'Ihis wfeatureresults in an automatic volume control that is more or less independentten cycles and'does not respond appreciably to static. The direction/ofcurrent ow in the rey sistor29 is such that` the end thereof connectedto the grid .of the intermediate frequency amplier tube acquires `anegative potential with respect to the end which is connected to thecathode and, as a consequence, the bias potential applied.

to the said intermediate frequency amplifier becomes more negative thannormal, the increase in the negative direction being proportional to theamplitude of the carrier wave impressed upon the demodulator tube.

It should be noted that the anode I l, of the tube I is, at all times,maintained somewhat negative with respect to the cathode thereof byreason of the drop across the resistor25. This is a desirable featuresince it prevents the automatic volume-control from functioning until asignal of predetermined volume is reached.

The self-bias rheostat 41 controls the maximum sensitivity and it may,with advantage, be manipulated during tuning to minimize backgroundnoise between stations. Preferably the resistance of the rheostat ishigh, of the order of y3000 ohms which may be adjusted to apredetermined minimum value for maximum sensitivity.

As hereinbefore explained, my improved auto- 'matic volume control sofunctions that the gain in the amplifier, forall signals above a.certain predetermined minimum value, is inversely proportional tocarrier amplitude. Suchbeing the case, further meansare required forcontrolling the sound loutput, from a. loudspeaker (not shown) to'meetdifferent requirements.

It is also desirable that, at low sound intensities, the highfrequencies shall be somewhat more attenuated than thelow frequencies.

Laccordingly, interpose a coupling network, for

obtaining automatically compensated volume' control, between theresistor -I3 included in the output circuit of the detector tube I andthe input terminals of an audio-frequency amplifying,I

tube 5I. The audio frequency amplifying tube may be ofthe equipotentialcathode type having an anode 52, a grid 53, and a cathode 55.- Aselfbias resistor 1,51v shunted by a condenser 59, is interposed betweenthe cathode 55 and ground and the grid 53 is' connected to ground.through a condenser 6I for additional manual tone-control purposeshereinafter tol be explained more 'in detail.1 y

The coupling network includes the resistance element 63 of apotentiometer, one end of which is connected to the grid 5 of thedetector tube 'through a stopping condenser 65 and a resistor 61 and theother end of which is connected to ground through a grid-leak 69. 'I'heresistor 61 and a by-pass condenser 10 serve to eiectively attenuate thecarrier frequencyiand prevent it from appearing on the grid of the tube5I Since the resistor 61 offers high impedance to the c'arrierfrequency,lit is compelled yto take a path to ground including theinductor 49 which offers vlow impedance to thet carrier frequency. The

junction between the resistance element .of the potentiometer and thegrid leak is connected to `the cathode through a condenser 1I which actsas a hum filter.

Inlorder that the potentials, at high frequencies, developed across thepotentiometer resistance element 63, shall be progressively attenuatedas the ymovable arm thereof vis moved down, in the direction givingreduced volume, separated points on the element are by-passed to thecathode over paths including individual condensers 15 and 11, individualresistors 19 and 8|, and the condenser 1I, previously mentioned.

It should be clear, from 'a consideration of the foregoing. that the lowfrequency potentials imthe grid'and the cathode.

them, are attenuated less than the high frequencies, thus giving thedesired effect. The degree of compensation depends upon the resistancebetween taps on the resistor 63 in relation to the condensers 15 and 11and the resistors 19 and 8|. If furtherattenu'ation of the highfrequencies is wished for, independently of the position of thepotentiometer arm 13, a resistor' 83, interposed between the arm and thegrid of the tube may be varied to control the shunting action of thecondenser 6| connected between A very small condenser 84 may beinterposed between the resistor 63 and the arm 13, if extremely highfrequencies are to be compensated to some extent. y,

By way of example, the resistor or impedance element 63 may have aresistance of 500,000 ohms and may be, and preferably is, whollyconstituted bythe resistance element of a potentiometer device. Thecondensers and 11 may havev values'of .0025 and .025 micofarad,respectively, y while the resistors 19 and 8| may have values of 100,000ohms and 10,000 ohms, respectively.

In order that the disclosure of my improved automatic volume controlsystem shall be vcomplete, I am giving below a listl of other moreimportant circuit constants:

Resistor Ohms Condenser Microfa'rads 13- 50,000 to 100,000 000750 2950,000 to 200,000 6l 0. 0001 67 20, 65 1 '83 0-1 megohm 70 .0002 84 0001Although I have chosen a 'specic embodiment of my inventien for purposesof illustration, many modifications thereof will be apparent to thoseskilled in the art to which it pertains. For example for a differentdegree of compensation,

different values of certain of the circuit come ponents as above givenmay bechanged. Particular reference is had to the circuit components -15and 11 together with the limiting resistors 19 -having terminal ends, aplurality of fixed spaced taps thereon between and spaced fromsaid'ends', means including electrical impedance elements Aprovidingseparate shunt connections between each of said taps and one end of saidresistor,

and means'providing at least one variable output tap connection movablealong said L.resistor between said ends. and successively past saidfirst named taps.

. 2. In an audio frequency volume control cir-l cuit to which signals ofsubstantially constant l average amplitude are applied, a singlevariable lvolume control means comprising an. impedancef-f`element1having terminal ends connected with said circuit and spacedtaps between said ends, and means in circuit separately between oneterminal and each of said taps for attenuating signals Within certainportions of the audio frequency range, said means thereby beingeffective with variations in volume, and being connected in shuntrelation to portions of said impedance element through said taps vandsaid one terminal.

3. In an audio frequency signal transmission circuit, means forsupplying audio frequency signals thereto 4at substantially-constantaverage amplitude, a potentiometer device having an impedance elementconnected in shunt with. said circuit, an output circuit connected withone ter-j diate the ends of said impedance element, said tap pointsbeing spaced apart along said impedance element progressively to provideattenuation progressively with decrease in volume by adjustment of saidcontact.

4. In an audio frequency signal transmission circuit, a poteniometerdevice having an impedance element connected in shunt with said circuit,

an output crrcuit connected with one terminal P and the variable contactof said potentiometer device and a plurality of signal attenuatingcircuits connected each with 4'said one terminal and a separate xed taplpoint on said impedance element, said tap points being spaced apartprogressively along said impedance element to progressively attenualesignals with decrease in volume by adjustment of saidcontact, and atleast one of saidcircuits including a condenser and a resistor connectedin series, thereby to attenuate signals in the -higher audio frequencyrange.

5. In an audio frequency signal transmission circuit, a potentiometerdevice having an impedance element connected in shunt with said circuit.an output circuit connected with one terminal and the variable contactof vsaid potenliometer device, and -a plurality of separate signalattenuating circuits conn'ected each between said one terminal and aseparate fixed tap point intermediate the ends of said impedanceelement, said tap points being spaced apart progressively along saidimpedance element and each of said signal attenuating circuits having alower impedance to a certain range of audio frequency signals than theimpedance of that portion Aof the potentiometer devicewith which it iseffectively connected in parallel.

6. In combination, a potentiometer device com- A,prising anI electricalimpedance element and a contact movable along said impedance elementto'provide a variable connection therewith, said impedance elementhaving a series of taps spaced progressively between its terminal ends,a plurality of separate signal attenuating frequency responsive meanseach connected with one of said terminal ends and with a separate one ofsaid taps, automatic volume control means for supplying audio frequencysignals of substantially constant amplitude to the terminals of saidimpedance element, and an output circuit connectedto j said movablecontact and one terminal end of said element, whereby movement of saidcontact toward said termina-l end provides volume red uction andprogressive signal attenuation.

7.121 combination. a potentiometer device cornh prising an electricalimpedance element and a contact movable along said impedance element toprovide a variable connection therewith, said impedance element havingtaps spaced progressively between-its terminal ends, a plurality of tappoints betweenand spaced from the of said resistance element. t

11. In an audio'frequency amplifier system, an4

separate signal attenuating means each connected with one of saidterminal ends and with a separate one of said taps, therebyprogressively to attenuate signals vas the contact is moved in thedirection of said one lterminal end, an audio frequency signal circuitconnected with the terminal ends of said impedance element, and an audiofrequency output circuit connected with said one of the terminal ends ofsaid impedance element and the movable contact.

8. In combination, apotentiometer device com-' prising 'an'electricalimpedance element and a contact movable along said impedance element toprovide a variable connection therewith, said impedance element havingspaced taps between its terminal ends, a plurality of signal attenuatingmeans each connected with one of said terminal ends and .with a separateone of said taps,

an audio frequency signal circuit connected with the terminal ends ofsaid impedance element, an audio frequency output circuit connected withsaid one of said terminal ends of said impedance eement and the movablecontact, and independent additional tone control means connected in saidoutput circuit between said contact and a high potential end of saidimpedance element.

9. In combination, a, potentiometer' device comi prising an electr/icalimpedance elementv and a` contact movablealong said pedance element toprovide a variable connect n therewith, said impedance element havingspaced taps between'I its ends, a plurality of signal attenuating meanseach connected with one kof saldends and with a separate one of saidtaps, an audio frequency signal circuit connected with the terminal endsof said impedance element, an audio frequency output circuit connectedwith said one of said terminal ends of said impedance element and themovable contact, and a high frequency lter insaid audio frequencyinput-circuit comprising a shunt coupling impedance, a series impedancein one side of said circuit, and a condenser connected in parallel withsaid circuit following said impedance'in the directionof signaltransmission.

10. In an audio frequency amplifier system, an audio frequency amplifierdevice having an output circuit, a second audio frequency amplifierdevice having an input circuit, and a volume control coupling network'interconnecting said circuits for progressively attenuating signals incer-v tain portions of the audio frequency range with decrease involume, said network comprising a potentiometerresistor having aresistance element connected in shunt to said output circuit,

a movable Acontact and one terminal connected with said input circuit,anda plurality of signal attenuating means connected to provide separatecircuits between said one terminal and spaced terminals audio frequencyamplier devicehaving an output circuit, a second audio frequencyamplifier device having an input circuit, vand a coupling networkproviding combined volume and tone control, interconnecting saidcircuits and comprising a potentiometer resistor having a resistanceelement connected yin shunt to said output circuit, and a plurality ofsignal attenuating device having an input circuit and a coupling lnetwork interconnecting said circuits and comprising a potentiometerresistor having a `resistance element connected in shunt to said outputcircuit, a movable contact and one terminal connected with said inputcircuit, and a plurality of signal attenuating means connectedv betweensaidone terminal and'spaced tap points thereon, and a separate variabletone control means connected with said input circuit.

13. In an audio frequency amplifier system, an

audio frequency amplifier device having an output circuit, a secondaudio frequency amplifier device. having an input circuit and acouplingnet- V'work interconnecting said circuits and comprising apotentiometer resistor having a resistance element connected in shunt tosaidfoutputcircuit, and a plurality of signal attenuating meansconnected between said one terminal and spaced tap points on saidresistance element, said signal attenuating means each comprising acondenser and a resistor connected in series with each other,

and a separate compensating circuit connection between the outputcircuit and the input' circuit,

said connection including a coupling condenser" of relatively lowcapacity. y

, 14. In an audio frequency volume control circuit, a volume controlpotentiometer device having a resistance element provided with highpotential and lo'w potential input terminals at its ends, a tapcontinuously movable along said element between said ends, a signaloutput circuit connected with said movable tap and the low potentialterminal, a signal input circuit connected with said high and lowpotential terminals, said element having a series of'taps spacedfromsaid high potential terminal and from each other, and a separatetone compensating circuit includinga react-ance element connectedbetween each of said last named taps 'and the low potentialterminal ofsaid first named element. i l

15. In an audio frequency signal `transniission circuit, a potentiometerdevice having an impedance element connected in shunt with said circuit,an output circuit connected with one terminal and the variable contactof said potentiometer device, and a plurality of signal attenuatingcircuits connected eachwith said one terminal and a separate fixed tappoint on said impedance element, said tap fpoints being spaced apartprogressively along said impedance element vto progressively attenuatesignals with decrease in volume by adjustment of\ said contact, and atleast one of said circuits including a condenser and a resistorconnected inv series, said circuit being connected with a tap pointwhereby it becomes effective in advance of said other circuits in theattenuation of said signals.. LOY E. BARTON.

